The most common reasons for lack of bloom on wisteria include too little sun, overfertilization with nitrogen (for example if the vine is growing next to a lawn that is heavily fertilized and the vine is also fertilized separately), improper pruning, winter damage or spring freeze damage to the flower buds, and finally and maybe most common, lack of maturity. Wisteria vines grown from seed can average about eight years before they bloom but a twenty year wait is not all that unusual. The following web page about growing wisteria may also offer some tips, it includes detailed information about pruning and training as well as overall care for these wonderful plants. http://ohioline.osu.edu/hyg-fa... I hope this helps you troubleshoot.